1. Field of the Invention
The present invention relates to a surface light source device of side light type, more specifically, relates to a surface light source device of side light type in which uniformity of luminance is improved. The present invention is applied to back lighting of a liquid crystal display device, for example.
2. Related Art
A surface light source device of side light type has been conventionally applied to a liquid crystal display device, for example, and this device illuminates a liquid crystal panel from a back surface. Its arrangement is suitable for thinning the whole form of the device.
In the surface light source device of side light type, a rod-type light source such as a cold cathode tube is employed as a primary light source, and it is arranged beside a light guide plate (plate-type member composed of a light guide material). An illumination light emitted from the primary light source is guided into the light guide plate through the side end surface of the light guide plate. The guided illumination light is propagated through the light guide plate, and according to this process, the light is emitted from a major surface of the light guide plate to the liquid crystal panel.
As the light guide plate employed in the surface light source device of side light type, a plate having substantially uniform thickness, and a plate whose thickness tends to become smaller with going away from the primary light source are known. In general, the latter plate emits an illumination light more efficiently than the former plate.
FIG. 3 is an assembly perspective view showing the surface light source device of side light type using the latter light guide plate. Moreover, FIG. 4 is a cross section taken along line A--A in FIG. 3. Referring to both the drawings, a surface light source device of side light type 1 has a light scattering guide plate (light guide plate composed of a light scattering guide material) 2, a primary light source 3 arranged beside the light guide plate 2, a reflection sheet 4 and prism sheet 5 as a directional characteristic correcting member. The reflection sheet 4, light guide plate 2 and prism sheet 5 are laminatedly arranged.
The primary light source 3 has a cold cathode tube (fluorescent lamp) 8 and a reflection member (reflector) 9 having an approximately semicircular cross section arranged in the circumference of the cold cathode tube 8. An illumination light enters the side end surface of the light scattering guide plate 2 through an opening of the reflector 9. As the reflection sheet 4, a sheet-like regular reflection member composed of metallic foil, etc. or sheet-like irregular reflection member composed of a white PET film, etc. are employed.
The light scattering guide plate 2 has a wedge-shaped cross section, and it is composed of a matrix composed of polymethyl methacrylate (PMMA) , for example, and transparent corpuscles which are dispersed uniformly in the matrix and whose refractive index is different from that of the matrix.
As illustrated by a zig-zag line in FIG. 4, an illumination light L from the primary light source 3 is guided into the light scattering guide plate 2 through an incidence surface 2A which is one side end surface of the light scattering guide plate 2. While the guided illumination light L is reflected repeatedly between a major surface 2B along the reflection sheet 4 (hereinafter, referred to as "slanted surface") and a major surface along the prism sheet 5 (hereinafter, referred to as "emitting surface") 2C, the illumination light L is propagated towards the end. At this time, the illumination light L is influenced by a scattering action due to transparent corpuscles. If the reflection sheet 4 composed of the irregular reflection member is employed, the illumination light L is influenced also by an irregular reflecting action.
As the illumination light L is reflected repeatedly on the slanted surface 2B, an incidence angle with respect to the emitting surface becomes smaller gradually. The decrease in the incidence angle increases a component of not more than a critical angle with respect to the emitting surface, and the emission from the emitting surface 2C is improved. As a result, insufficient emitting light is prevented in a region far from the primary light source 3.
Since the illumination light emitted from the emitting surface 2C has been scattered by the transparent corpuscles or has been reflected irregularly by the reflection sheet 4, it has a property of scattered light. However, the main propagating direction of the emitted illumination light is slanted to an end direction with respect to the emitting surface (opposite direction to the primary light source 3). Namely, the emitting light has directivity. The surface light source device of side light type 1 using such a light guide plate generates an illumination light having a directional emitting characteristic.
The prism sheet 5 is formed by a transparent sheet material such as polycarbonates, and the prism surface is formed on both of the surfaces. The prism sheet 5 is arranged so that its prism surfaces face towards the light scattering guide plate 2. The two prism surfaces are composed of a plurality of convex portions having rectangular cross sections which are extended approximately parallel with or vertically to the incidence surface 2A of the light scattering guide plate 2. The extending directions of the convex portions on both the prism surfaces intersect perpendicularly to each other, but they are occasionally parallel with each other.
The prism sheet 5 corrects the main emitting direction of the emitting light to a frontal direction of the emitting surface on the slanted surface of each convex portion. A prism sheet having the prism surface only on one side is occasionally employed. In this case, the extending direction of the convex portions is generally parallel with the incidence surface 2A.
In general, the surface light source device of side light type 1 employing such a wedge-shaped light guide plate emits an emitting light to the frontal direction more efficiently than the surface light source device of side light type employing the light guide plate having substantially uniform thickness.
Here, the surface light source device of side light type can adopt a light guide plate in which a scattering film, rough surface, etc. are formed on one or both of the major surfaces (emitting surface and/or rear surface) of a wedge-shaped or wedge-like-shaped transparent and semi-transparent member as the light guide plate having the directional emitting characteristic. Such a light guide plate also emits an emitting light to the frontal direction efficiently.
The surface light source device of side light type 1 employing the light guide plate having the aforementioned directional emitting characteristic causes an unsolved problem such that nonuniformity of luminance occurs on the emitting surface 2C. The nonuniformity of luminance, as shown in FIG. 3, occurs in a form of bright band in which the luminance level is relatively high and dark band in which the luminance level is relatively low. The bright-and-dark-band contrast tends to appear in the vicinity of the incidence surface 2A so as to be substantially parallel with the incidence surface 2A.
According to a method to solve the above problem, a light diffusible surface is formed on the incidence surface 2A. However, this method cannot restrain the bright-and-dark-band contrast sufficiently in some forms of the light scattering guide plate.